1. Field of the Invention
The present invention relates to a fixing unit incorporated in an electrophotographic recording apparatus, and more particularly to a fixing unit where the developer material deposited on a print medium is pressed and heated to fuse.
2. Description of the Related Art
A conventional fixing unit for use in an electrophotographic printer includes a rotating heat roller and a rotating backup roller. The surfaces of the heat roller and backup roller have a rubber material or a resin material wrapping around them. The heat roller is cylindrical and has a built-in heater in the form of, for example, a halogen lamp. Electric power is supplied to the heater, which in turn generates heat to heat the heat roller to a desired temperature.
The heat roller has a temperature sensor in the form of a thermistor. The temperature sensor detects the temperature of the surface of the heat roller. The detection signal causes a control circuit to turn on and off the electric power supplied to the halogen lamp, thereby maintaining the surface temperature of the heat roller to a substantially constant value. For safety of the system, there is provided a thermostat that shuts off electric power when the feedback control operates abnormally to overheat the heat roller. The thermostat shuts off the electric power before the temperature of the heat roller exceeds a maximum allowable value, thereby preventing an abnormal increase in temperature.
FIG. 6 illustrates another conventional fixing unit. Referring to FIG. 6, instead of a heat roller and a backup roller, a fixing unit 1 uses two heat rollers 2 and 3 that heat a print medium both from the front side and from the back side simultaneously. This type of fixing unit is advantageous when the printing speed of the electrophotographic printer is to be increased. The recording medium passes through the fixing unit at a high speed and therefore heat rollers must apply a sufficient amount of heat to the printing medium in a short time during which the printing medium passes through the fixing unit.
There are provided thermistor sensors 4 and 5 on the heat rollers 2 and 3, respectively. The thermistor sensors 4 and 5 are connected to control circuits 18 and 19 through connectors 6 and 7, respectively. The control circuits 8 and 9 are connected to an a-c main line 12 through thyristors 10 and 11.
Thermostats 13 and 14 are disposed on the surfaces of the heat rollers 2 and 3, respectively. The thermostat 13 has one cord connected to a halogen lamp 15 in the heat roller 2 and the other cord connected to the a-c main line 12 through a fuse 16 and the connector 6. Likewise, the thermostat 14 has one cord connected to a halogen lamp 17 in the heat rollers 3 and the other cord connected to the AC main line 12 through the fuse 16.
FIG. 7 is a graph that illustrates changes in the surface temperature of the heat rollers in the conventional art when temperature control fails. Curve A indicates the surface temperature of the heat roller 22 when temperature control fails and Curve B shows the surface temperature of the heat roller 23 when temperature control operates normally.
The operation of the conventional fixing unit of the aforementioned construction will be described. If the temperature control involving one of the thermistor sensors 4 and 5 should fail and a heat roller in a failed control system is overheated, a corresponding thermostat operates to shut off the circuit before the temperature reaches a tolerable value. The temperature of the heat roller 23 under abnormal temperature control will start to decrease. The heat roller 22 under normal temperature control maintains its surface temperature substantially at the target value. Therefore, as shown by Curve A, the temperature of the heat roller 23 will not decrease rapidly but slowly reach the target value at time t3.
With the aforementioned conventional fixing unit that employs two heat rollers, the thermostats are connected to separate circuits. If one of the feedback control systems fails, a corresponding thermostat in the failed system is shut off. A thermostat in the normally operating system is not shut off but performs its on and off operation under the control of the output of a corresponding thermistor. In other words, the surface of a normally operating heat roller is maintained at a desired temperature. The temperature of an abnormally operating heat roller will not decrease and the abnormal condition will remain for a long time.
An object of the invention is to provide a fixing unit in which when a heat roller is overheated due to an abnormal condition, the abnormal condition is prevented from lasting for a long time.
A fixing unit fixes a developing material deposited on a recording medium by heating and pressing the recording medium. The fixing unit includes a first number of heating members that apply heat to the recording medium and a second number of switches that supply electric power to the first number of heating members. Each of the heating members receives electric power through a series circuit of the switches. Each of the switches responds to a surface temperature of a corresponding one of the heating members so that when the surface temperature exceeds a predetermined value, the electric power is shut off.
The first number of heating elements may be in parallel with each other.
The first number is equal to the second number.
The first number of heating members may include two heating members and the second number of switches may include two switches. The two switches form a series circuit with each other and the two heating members form a parallel circuit with each other. The series circuit is connected in series with the parallel circuit.
The first number of heating members may include three heating members and the second number of switches may include three switches. The three switches form a series circuit with each other and the three heating members form a parallel circuit with each other. The series circuit is connected in series with the parallel circuit.
The first number of heating members may include two heating members and the second number of switches may include four switches. A first one of the two heating members forms a first series circuit with first two of the four switches. A second one of the two heating members forms a second series circuit with second two of the four switches. The first series circuit and the second series circuit are connected in parallel with a power source.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.